Various types of thermoplastic elastomer compositions containing elastomers, both cured and uncured, and thermoplastic resins, are known in the industry as either thermoplastic plastic vulcanizates (TPVs) or as dynamically vulcanized alloys (DVAs). The elastomer is dispersed in the thermoplastic resin, providing flexibility to the material due to the elastomer and reprocessability due to the thermoplastic resin. These materials are known to be useful in a variety of applications including automotive parts, such as bumpers, knobs, and trim, electrical and applications, such as cable jacketing and connectors, and industrial applications, such as piping, o-rings, sleeves, extruded spiral hoses, and weather stripping. For all of these known applications, the TPVs or DVAs are cast or molded to form the final products.
The conventional fabrication process is a multiple-step process having the following steps. The compound is produced by (i) preparing a rubber master batch by mixing, at temperatures below the cross-linking temperature, the elastomer and curative until a uniform mix state is obtained (this is often referred to as pre-conditioning) and (ii) premixing a resin master batch comprising a thermoplastic resin and plasticizers. If desired, fillers such as carbon, oil, calcium carbonate, nanofillers, etc., may also be added to the rubber master batch. A thermoplastic resin masterbatch is mixed typically in a twin screw extruder by mixing the resin and plasticizers. The resin masterbatch may then be pelletized. The rubber master batch, resin master batch, and all remaining components are then fed into a mixer, as well as any desired secondary components, and mixed under shear conditions. The elastomer component is vulcanized during the melt mixing.
Commercial TPVs are typically not compounded or prepared for use in low-permeability applications, and are generally polyolefin based compounds. The processes existing to produce the polyolefin based TPV are operated at high extruder screw speeds (>greater than 250 revolutions/minute) and corresponding high peak shear rates. The higher screw speeds allows for high net output. See U.S. Pat. No. 5,298,211 and U.S. Pat. No. 4,594,390. The processes utilize the high shear rates to reduce rubber particle size, simultaneously during the curing reaction in the extruder. The typical rubber particle size for such thermoplastic elastomers is above 1 micron size, often 3 to 10 microns. An energy efficient process for producing sub-micron size particles is different from the above noted patents due to the fact that the basic rubber particle structure is thought to be formed in part due to an interfacial reaction between the rubber and the thermoplastic prior to cure.
DVAs compounded for low permeability [or stated alternatively: high impermeability] applications comprise low-permeability thermoplastic resin, such as polyamide or a blend of polyamides, in which there is dispersed a low-permeability rubber. Such low permeability rubbers include butyl rubber, halobutyl rubbers, or brominated isobutylene para-methylstyrene copolymers. The rubber is cured under conditions of dynamic vulcanization (curing the rubber during melt mixing as opposed to static curing that typically occurs in a rubber mold) and is intimately and uniformly dispersed as a particulate phase within a continuous phase of the thermoplastic resin. For low permeability applications, it is desired to achieve a composition having sub-micron size dispersed rubber particles. This dispersed particle size assists the material in having elastic properties.
The elastic nature is desirable for applications requiring flexibility, strength, and elongation. Such properties are also desirable in tire materials. Thus, in recent years, the use of DVAs as tire inner liner layers has been explored. The thermoplastic resin provides a very low permeability to the inner liner layer while the elastomer provides flexibility and durability to the inner liner layer. As the thermoplastic resin provides a very low permeability, in comparison to an all elastomeric inner liner composition, the inner liner layer formed from DVA can be formed as a very thin layer. Conventional inner liner layers, comprised of only a base elastomer(s), typically have a thickness or gauge in the range of 1.25 to 7.0 mm while inner liner layers formed from DVA have typically a thickness range of 0.25 mm to 0.08 mm.
However, the past work of Applicants and others in using DVA for tire innerliners has highlighted the need for continued improvement in the process of preparing DVAs. As noted above, TPVs and DVAs have conventionally been molded or cast to form the end products. Films having a thickness in range of 0.25 mm to 0.08 mm are not molded or cast, but must be extruded or blown thru a suitable die. The morphology of the DVA material, which is impacted by the DVA manufacturing process, has a significant impact on the ability to obtain a quality extrusion and quality film. In particular, a DVA film's low temperature durability is dependent not only on the composition but also on the morphology of the final product.